Magnetically Actuated Drops‐on‐Stilts Microrobots for Transport and Delivery of Hydrophobic Cargo

ABSTRACT Magnetically actuated microrobots assembled from iron oxide nanoparticles (IONPs) can be utilized for drug‐delivery applications, yet efficient transport of hydrophobic cargo remains challenging. We introduce “drops‐on‐stilts” microrobots, formed via co‐assembly of ferromagnetic IONPs and oil droplets under uniform rotating magnetic fields. Assembly requires a particular wetting regime; IONPs must be partially wet by oil, and be present in bulk suspension and at the droplet interface below the interfacial jamming limit. By varying the ratio of IONPs to oil droplets, we generate distinct structures. At low IONP ratio, interfacial assemblies with linear dimension smaller than the droplet diameter concentrate at droplet poles, enabling rotation. At higher IONP ratio, IONP chains merge with interfacial particle assemblies to form stilts longer than the drop diameter that lift droplets, enabling a walking motion at speeds that scale linearly with stilt length. High droplet densities promote chaining into “log‐like” collectives capable of rolling or walking, transporting hydrophobic payloads over irregular obstacles. Monte Carlo simulations modeling IONPs as adhesive dipolar hard spheres reproduce stilt‐like chain formation and bundling, validating the key mechanisms for structure formation. Drops‐on‐stilts magnetically guided delivery of the essential oil eugenol in the targeted treatment of Candida albicans biofilms is demonstrated.